Preterm birth is a significant health problem in the U.S. resulting in a large economic burden to society and a major impact on quality of life for survivors and their families. Surviving extremely low birth weight (ELBW) infants born at less than 1000 grams have a high rate of neurodevelopmental delay. During the neonatal period, ELBW infants require blood transfusions due to frequent blood sampling and an immature bone marrow response. The ideal hemoglobin level at which to transfuse is not known and there are conflicting data as to whether targeting a higher hemoglobin improves neurodevelopmental outcome. Practices remain widely divergent in the absence of compelling data regarding risks and benefits. Transfusion of Prematures (TOP) (1U01HL112776-1), currently underway at the 18 clinical centers of NICHD's Neonatal Research Network (NRN), is a randomized clinical trial designed to determine whether the primary outcome of death or significant neurodevelopmental impairment (NDI) in survivors at 22 to 26 months is less common among preterm infants maintained at a higher hemoglobin level. The following proposal is submitted in response to the Ancillary Studies in Clinical Trials RFA by Valerie Chock, MD MS, an early stage investigator with expertise in the use of near infrared spectroscopy (NIRS) in preterm infants. NIRS is a non-invasive bedside technology able to reliably measure regional tissue oxygenation of both cerebral and mesenteric tissues. Regional tissue oximetry may be a better measure of end-organ oxygenation than hemoglobin alone and may provide physiologic data predictive of NDI as well as necrotizing enterocolitis (NEC), an adverse outcome potentially related to transfusion in ELBW infants. One objective of this proposal is to determine differences in cerebral oxygenation and fractional tissue oxygen extraction with NIRS (Aim 1) between high and low hemoglobin threshold groups during red blood cell transfusions. We also propose to determine whether abnormal cerebral NIRS measures are a better predictor of NDI than hemoglobin alone (Aim 2) and whether abnormal mesenteric NIRS measures are associated with the development of NEC within the 48 hours following a transfusion (Aim 3). Eligible infants enrolled in the TOP trial will have cerebral and mesenteric NIRS monitoring during the first 7 days of life and during subsequent transfusions. With an estimated sample size of 490 infants, longitudinal analyses will determine the association between NIRS measures and transfusion threshold group. Classification and Regression Tree (CART) analyses will determine the predictive capability of abnormal NIRS measures for the outcome of death or NDI and the outcome of transfusion-associated NEC. The feasibility and success of this proposal is enhanced by collaboration with the TOP PIs and commitment by NRN centers to participate. The novel use of regional tissue oximetry will provide physiologic measures critical to understanding outcome differences in the TOP trial. This ancillary study will significantly improve knowledge of the benefits and risks of transfusions in the preterm neonate, resulting in optimized transfusion practices.